Epithelial-mesodermal interactions in digit patterning and evolution

Pattern formation in the limb requires communication between two principal signalling centres: the apical ectodermal ridge (AER) in the epithelium and the polarising region (PR) in the mesoderm. However, understanding the individual contributions of these signalling centres in development is very difficult in vivo, as the AER is vital for the outgrowth of the limb. Studies in the chick wing have suggested that Sonic hedgehog (Shh) signalling intrinsically regulates the rate of proliferation in PR cells via an autoregulatory cell cycle mechanism which prevents it from forming posterior digits, a characteristic seen in the mouse/mammalian limb.

In my thesis, I have dissected and cultured PR tissue without the AER or anterior mesenchyme to develop a PR explant model to understand the relative roles of these organisers. By examining the duration of Shh signalling and proliferation parameters, I determine that the PR maintains in vivo parameters in the absence of the AER. Attenuating Shh signalling in the PR explants reveals that it inhibits proliferation through a Bmp2-p27kip1 pathway to prevent the formation of an additional digit. Interestingly, in this process, my results reveal that the PR explants display an unexpected ability to self-organise and re-establish a population of AER-like cells.

Finally, analysis of cell cycle regulators in the mouse PR indicate that the avian wing PR has evolved the Bmp2-p27kip1 pathway to prevent the formation of PR-derived digits. I present a potential evolutionary mechanism to explain how posterior digits were lost in the evolution of the bird wing.